1. Field of the Invention
The present invention relates generally to amplifier circuits, and more particularly to an apparatus and method for attenuating an undesired frequency in a signal output from an RF amplifier circuit in a wireless communications system, for example.
2. State of the Art
Wireless communications systems may typically include a chain of amplifier circuits in stages, each stage comprising an amplifier an filter circuit through which a received or modulated signal is passed in series. At each stage the filter circuit filters out unwanted (out of band) frequencies while the amplifiers amplify the remaining signals. Typically, each stage in the chain of amplifier circuits is a Radio Frequency (xe2x80x9cRFxe2x80x9d) amplifier circuit.
RF amplifier circuits are well known and widely used in, for example, receivers, transmitters and transceivers including devices such as cellular telephone handsets, base stations, pagers and wireless modems.
An example of an RF amplifier circuit suitable for use in a cellular telephone handset is shown in FIG. 1. Referring to FIG. 1, a conventional RF amplifier circuit 10 typically includes an amplifier 12 having at least one active element or device 14 for amplifying a desired frequency or frequencies in a signal received on an input 16 thereto, and a network 18 performing the dual role of impedance transformation and suppressing or attenuating an undesired frequency or frequencies in the signal from an output 20 thereof. In the example shown in FIG. 1, the filter 18 is a particular type of low-pass filter (LPF), known as a three-section LPF, having a shunt capacitor 22, a series inductor 26 and another shunt capacitor 24, connected in that order. Values of the capacitors 22, 24, and the inductor 26 are selected to pass substantially unimpeded all frequencies below a predetermined first frequency (f0) while attenuating all frequencies above f0. For simplicity the amplifier circuit 10 is shown as including a single amplifier 12 with a single active element 14 and a single filter 18, however it will be appreciated that the amplifier circuit can include additional active elements and filters.
A graph of the output versus frequency of the amplifier circuit 10 of FIG. 1 is illustrated in FIG. 2. FIG. 2 is a graph of the gain, that is the change in strength of the signal between the input 18 and the output 20, versus frequency. As shown by line 28 in FIG. 2, the amplifier 12 is biased and the filter designed such that all frequencies below f0 have a generally constant gain and are passed through the filter 18 substantially unimpeded, while all frequencies greater than f0 are attenuated by an amount or factor that increases in proportion to the frequency. Generally, it is desirable to suppress the undesired frequencies above f0 to avoid distortion of the desired output waveform.
A particular problem with convention amplifier circuits is the suppression of harmonics of the desired frequency, and more particularly the suppression of a second harmonic of a desired or fundamental frequency. Because of the proximity of the second harmonic, 2f0, to the fundamental frequency, f0, conventional amplifier circuits using simply a low-pass filter have generally been unable to sufficiently suppress the second harmonic to avoid signal distortion. For example, as shown in FIG. 2, for an amplifier circuit using a conventional LPF 18 as shown in FIG. 1, the signal out will include in addition to the fundamental frequency a second harmonic that is attenuated by a factor of less than about 30 dB relative to the fundamental.
Several approaches have attempted to provide an amplifier circuit having a filter or apparatus for sufficiently suppressing the second harmonic while pass the fundamental frequency substantially unattenuated. One approach, also shown in FIG. 1, is the addition of a series-resonant trap 30 in shunt with the output 20 of the amplifier circuit 10. The series-resonant trap 30 is designed to have a low impedance to any frequencies occurring at the second harmonic, thereby shunting a portion of this component of the signal to ground. The result, as shown by line 32 in FIG. 2, is a dip, or notch, in the output from the amplifier circuit 10 at the second harmonic. However, while this approach is effective to a degree, it is not wholly satisfactory. For example, to generate 1 W of power from a 3 V battery requires the active device to have load impedance below 4 ohms. For the shunt trap to successfully remove signals at the second harmonic, its impedance must be significantly lower, such as 0.5 ohms. Such low impedances are difficult to attain.
Another problem with the use of the series-resonant trap of FIG. 1 is the impact of the trap on other characteristics of the circuit.